Plot-scale runoff experiments play an important role in soil and water conservation monitoring, which can not be replaced by other measures. In order to ensuring the accuracy and reliability of the data from the plot scale measurements, it is crucial to finding the most suitable methods regarding monitoring and data analyzing procedures. In the semiarid environment，recognization of rainfall event is significant for distinguishing different runoff events caused by natural rainfall and recording the related hydrological processes simultaneously, especially in consecutive rainy days. However, how to define one single rainfall event in the real world? How long the interval between two parts of rainfall is considered as an independent events How to define the corresponding runoff and erosion caused by such rainfall events? All these important problems need to be solved. The displacement method is tried for treating water samples. The method is relative economical and effective, only need some pycnometer, and the data quality can be ensured. Therefore, we sug- gest using this method，especially in the high erosive regions. Meanwhile, the citation of material and uzilization of information from runoff-plot measurements should be carefully amended and verified, and can not be used without any verification.